FIG. 1a shows a schematic circuit diagram of a first known type (T1) of fire detecting device, which includes an ionisation chamber C for detecting smoke. In this type T1 of detector, a threshold voltage of a zener diode ZD1 is selected according to the parameter which is sensed, e.g. smoke, heat, flame or some other parameter. FIG. 1b is a circuit diagram of another type T2 of fire detecting device, i.e. a manual call point which includes a switch 3 in series with a zener diode ZD2. Both of these devices can be connected via terminals L1, L2 to a pair of supply lines 7 (shown in FIG. 2) to which is also connected a central control unit (CCU) 6, of conventional construction. Each type T1,T2 of detecting device applies a high impedance across the supply lines 7 in a standby condition, but is responsive to a change in state, due to a fire condition, to apply a low impedance across the supply lines 7 in an alarm condition. The CCU 6 provides a current supply on the lines at a voltage which is higher than the threshold voltage of the zener diodes ZD1, ZD2, the supply being current limited by a resistor (not shown), or other means to prevent the power dissipation rating of the zener diodes being exceeded in an alarm condition.
In FIG. 1a, resistor 4 is shown connected in parallel with zener diode ZD1, so as to maintain thyristor 5 in a latched state, in the event that the supply voltage becomes less than the threshold voltage of the zener diode ZD1. This may occur when a second detecting device (not shown), connected to the same line 7 and having a slightly lower threshold voltage zener diode, also detects a fire.
FIG. 2 is a schematic diagram of a known fire detection and alarm system in which CCU 6 is shown connected via lines 7 to a first type of detecting device T1 (similar to that shown in FIG. 1a) and a second type of detecting device T2 (similar to that shown in FIG. 1b). Alarm devices 10 are shown connected to the CCU 6 by a second pair of supply lines 8. The CCU 6 includes voltage threshold sensing means (not shown) responsive to the voltage across lines 7, i.e. at points A and B, and switching means (not shown) responsive to the voltage threshold sensing means which activates the alarm devices 10 to produce an alert or evacuate warning, according to the voltage across lines 7, when a fire has been detected. In this alarm condition, the detecting devices T1, T2 provide different low impedances across the lines 7 which limit the voltage across lines 7 to different voltages determined by, for example, the use of zener diodes with different zener voltages in the detecting devices T1, T2.
FIG. 3 shows a known current/voltage characteristic 11 of the first type of detecting device T1 (e.g. a smoke detector). Characteristic 12 is that of a second type of detecting device T2 (e.g. a manual call point). These characteristics would be obtained when either device T1 or T2 is in an alarm condition. FIG. 3 also shows an example of a load-line characteristic of the output of the CCU to the fire detection devices. Characteristic 13 is that of a supply which is derived, for example, from a voltage source of 24 volts connected in series with a 600 ohm resistor. In the alarm condition, the voltage across the supply and signalling lines is the voltage at which the characteristic of the fire detecting device intersects the CCU load-line. In the standby condition, the voltage across A and B is 24 volts. When only the first type of detecting device T1 (the smoke detector) is in the alarm condition, the voltage across A and B is 15 volts and the CCU switches the alarm devices 10 to signal an "alert". However, whenever the second type of detecting device T2 (a manual call point) is in the alarm condition, the voltage across A and B is reduced to 10 volts and the CCU switches the alarm devices to signal "evacuate".
A disadvantage of the system described above is that separate lines are needed for fire detecting devices and alarm devices. This is because detecting devices in the alarm condition would be damaged by the high current available from the supply applied to the lines to operate alarm devices.
Our copending UK application No. 9808094.8 (corresponding to U.S. Application Ser. No. 09/292,199), to which reference may be made for further details, discloses a detecting device comprising signalling means for producing a change of state signal, from a quiescent state to an alarm state, when a change in condition or environment occurs, no such change normally occurring in the quiescent state. The detecting device also has impedance switching means with high and low impedance states which are applied across terminals for connection to supply lines. The impedance switching means is normally in a high impedance state when the signalling means is in its quiescent state, but responds to a change of state signal, from the signalling means, so as to switch to a low impedance state. The low impedance state increases current drain on the supply lines so that it is recognisable as a fire detection signal. The detecting device normally operates with a first voltage present on the supply lines. However, an alarm device is actuated when a second voltage, higher than the first voltage, is applied to the supply lines. The detecting device further includes voltage responsive means, which respond to the second voltage, to cause the impedance switching means to switch to a high impedance state, so that the line impedance across the supply line terminals is increased, so as to reduce current drain and thereby conserve power. Thus, when the alarm condition exists, the alarm device can be provided with a relatively high operating current at the second voltage, in order to give a fire warning, but the current drain by the detecting device is reduced to a minimum, to conserve available power. This is important when the power supply is a battery, because it can extend the life of the battery under the alarm condition.
In a preferred embodiment disclosed in the same copending UK 9808094.8 (corresponding to U.S. Application Ser. No. 09/292,199), means are provided for generating an "alert" signal when a smoke detector is in an alarm condition, and an "evacuate" signal whenever a manual call point is in an alarm condition. A disadvantage of the means disclosed in the copending application is that the identification of the signal from a manual call point involves reversing the polarity of the voltage applied to the supply lines, thereby increasing CCU complexity and cost. A further disadvantage is that polarity must be observed when connecting smoke detectors to the supply lines and this can lead to errors during installation.
In other systems, such as that described in GB2178878 and generally known as analogue addressable fire detection and alarm systems, a digital communications protocol is used by the fire detecting devices to signal to a CCU a code which identifies the type of detecting device that is transmitting a fire detection signal and a change of the parameter which is being monitored (e.g. smoke). The CCU in such a system can use the communications protocol to send signals to alarm devices which are activated according to the type of detector that has signalled the change in parameter being monitored. However, analogue addressable systems require more complex and more expensive electronics in detecting devices, alarm devices and the CCU, than in conventional, i.e. non-addressable systems which use conventional, non-addressable fire detecting devices, having only two or three operating states.
Despite various attempts, in the past, to solve problems of giving priority to a signal from a particular type of fire detector over a signal from another type of fire detector, in systems where fire detectors and alarm devices are operated on the same two wire supply, no satisfactory solution has been found. At least in its preferred embodiments, the present invention provides a solution to this problem which has the advantage of simple and low-cost construction and which can also employ robust electronics in the circuitry of the fire detectors, alarm devices and CCU.